In Paris on Thursday, riot police fired teargas in clashes with youths in the Invalides areas near the foreign ministry, witnesses said.
Youths threw stones at the police and set fire to the door of a block of flats in running battles at the end of a largely peaceful rally by thousands of students and workers against the CPE First Job Contract.
Aides said Dominique de Villepin, the prime minister, would meet senior trade union officials on Friday to try to defuse the crisis that has triggered a national strike threat and drawn hundreds of thousands of protesters on to the streets.
Charlie Herblin, a 22-year-old worker on the march, said: "This time, there are lots of young criminals on the march who are there to steal and smash. This discredits the movement."
Dozens of young people, many wearing masks or hoods, overturned cars, smashed shop windows and robbed demonstrators of clothes and mobile phones, witnesses said. The police said they had arrested 42 people.
Clashes also broke out in Rennes, where about 300 to 400 youths fought police.
Tens of thousands of students marched in cities throughout France, including Tours, Orleans and Marseille, as part of rolling protests designed to maintain pressure on de Villepin to axe a contract they say will create "Kleenex workers" that employers can throw away at will.
Unions have called a national strike for Tuesday to demand the withdrawal of the CPE, which allows employers to fire people aged under 26 at any stage during a two-year trial period without stating a reason.
In response to a written invitation to a meeting from Villepin, leaders of the five main labour confederations said they would meet the prime minister on Friday, but they reiterated their demand for the CPE to be withdrawn.
Jacques Chirac, the president, has increased pressure on his prime minister to renew contact with unions, said the Le Parisien newspaper, suggesting that de Villepin's job was now at stake.
An injured protester is taken
away by firemen in Invalides
"If things don't change very quickly, the prime minister will be fired," it quoted one government source as saying.
De Villepin's popularity has slumped and analysts say the protests are damaging the prime minister's ambitions to run in the 2007 presidential election.
Responding to prodding from Chirac, de Villepin told union leaders in his letter that he wanted a meeting as quickly as possible and would not restrict the agenda. Until now he has said he would discuss only how best to enforce the new law.
In the letter, de Villepin said he wanted to discuss "without preconceptions" ways to tackle the fears raised by the CPE.
De Villepin has championed the law as a tool to cut youth unemployment now running at 23%. Ministers have offered to halve the trial period and require bosses to justify layoffs.
Protests have disrupted three-quarters of the country's 84 universities and Gilles de Robien, the education minister, said exams could be postponed until the autumn.
Nicolas Sarkozy, the interior minister, de Villepin's rival to contest the presidency under the conservatives' colours, broke ranks over the CPE contract on Wednesday, calling for it to be given a six-month trial period.
Sarkozy fears that the protests could boost the left and sink the conservatives' chances in both the presidential and parliamentary polls due next spring.